1. Field of the Invention
The invention relates to a method and a device for producing coated strands of metal, in particular, of strips of steel, wherein a metal strand is guided through the bottom of a vessel, filled with a molten mass of identical or different composition as the metal strand, wherein the residence time of the metal strand is selected as a function of the molten bath level, the casting speed, the metal strand thickness, and the preheating temperature of the metal strand such that the deposited molten mass on the metal strand provides a desired thickness of a multiple of the initial thickness of the metal strand and the metal strand with the layer partially crystallized thereon is subjected to a smoothing pass after exiting from the molten bath which is performed when the surface temperature of the partially crystallized strand is lower than the solidus temperature of the molten bath and thus at least the surface of the partially crystallized layer is solidified.
2. Description of the Related Art
In a method and device known from EP 0 832 990 A2 a disadvantage in the production of coated metal strands, preferably strips of one steel quality or different steel qualities, such as, for example, single material or composite material and in this connection particularly also composite material of carbon steel, finely coated with stainless steels, is eliminated. A disadvantage in such inversion casting devices for a partially crystallized layer with a xe2x80x9cdoughy surface and doughy corexe2x80x9d was found to be that the layer adhering to the mother strand, on the one hand, has solidified already to a relatively large degree but, on the other hand, its outer zone still contains sufficient proportions of liquid phase when entering the smoothing roller pair. The strand is then subjected during its pass through the smoothing roller pair to a great supercooling so that there is a tendency of crack formation in the longitudinal as well as transverse directions of the strip. This risk increases even more for higher casting and rolling speeds. Accordingly, in this situation, the smoothing pass is performed when the surface temperature of the crystallized strand is smaller than the solidus temperature of the molten bath and thus at least the surface of the crystallized layer is solidified. However, it still cannot be prevented that the thickness of the partially crystallized layer fluctuates.
The invention has therefore the object to provide a method and a device which ensure smoothing of the strip with a strip thickness tolerance of a maximum of 2% without crack formation in the surface as well as in the interior of the strip with simultaneous thickness control of the partially crystallized layer.
In accordance with the present invention, a method for producing coated strands of metal includes guiding a metal strand through the bottom of a vessel filled with a molten mass of the same or different composition as the metal strip, wherein the residence time of the metal strand is selected as a function of at least one of the molten bath level, the casting speed, the metal strand thickness, and the preheating temperature of the metal strand such that the deposited molten mass on the metal strand has a desired thickness of several times the initial thickness of the metal strand. After exiting from the molten bath, the metal strand with a layer crystallized thereon is subjected to a smoothing path carried out when the surface temperature of the strand crystallized thereon is smaller than the solidus temperature of the molten bath, so that at least the surface of the layer crystallized thereon is solidified. The crystallized layer is applied with a thickness which exceeds the desired final thickness of the coated strand. Between the molten bath and the beginning of the solidification while still in the doughy state of crystallization, stripping of the crystallized layer is carried out to a controlled uniform smaller thickness over the length of the strand.
The main features of producing flawless, planar-coated strips, for example, of a width/thickness ratio of greater 60 and a total thickness of a maximum of 12 mm, preferably 2 to 6 mm, of one material or composite materials of different metal qualities such as carbon steel as a single material or carbon steel with a stainless steel coating of at least 5% of the total strip thickness as a composite material and a thickness fluctuation of a maximum of 2% between the edge (40 mm from the edge) and the center of the strip, are characterized with respect to the desired goal of a uniform thickness of the partially crystallized layer particularly in that, between the molten bath and the beginning of solidification on the strip surface, a stripping off of the partially crystallized layer of a desired thickness to a smaller thickness is carried out. The final effect thus is that only that much material is stripped off or removed as is required in order to obtain a uniform thickness of the partially crystallized layer across the length of the strand. Accordingly, the excess of the partially crystallized layer is stripped off.
Stripping off the partially crystallized layer with a desired thickness to a smaller thickness is carried out according to the device advantageously between the molten bath and the beginning of solidification on the strip surface, wherein the walls of the chamber receiving the smoothing roller pair as well as the smoothing rollers themselves are embodied to be heat-controlled. For stripping off, mechanical stripping devices in the form of preferably blades are advantageously provided; alternatively, a pneumatic stripping device can be employed.